Python: Deprecate the legacy memory APIs - #10273

Motivation & Context

The older memory classes overlap with the simpler FileMemoryProvider. This
change marks them as deprecated while keeping existing applications and their
saved memories working.

Description & Review Guide

• What are the major changes? Mark MemoryContextProvider,
  MemoryFileStore, MemoryStore, MemoryIndexEntry, and MemoryTopicRecord
  as deprecated. Keep their experimental notices and exports, document
  DEFAULT_MEMORY_SOURCE_ID as legacy, and add compatibility tests.
• What is the impact of these changes? Existing imports do not warn; using
  or subclassing the old classes produces a deprecation warning. The old
  provider keeps its automatic memory extraction, transcript history, and
  storage layout. The default harness provider remains FileMemoryProvider.
  The new provider lets the agent manage memory files, uses a different layout,
  and does not automatically extract memories or provide transcript history.
  Existing files are not moved automatically. The new provider uses a session
  scope by default; callers can supply a scope to share memory across sessions.
• What do you want reviewers to focus on? Warning locations, custom
  MemoryStore subclasses, and compatibility with existing saved memories.

Before:

from agent_framework import MemoryContextProvider, MemoryFileStore

provider = MemoryContextProvider(
    store=MemoryFileStore("./legacy-memory", owner_state_key="user_id")
)

For new file-based memory:

from agent_framework import FileMemoryProvider, FileSystemAgentFileStore

provider = FileMemoryProvider(store=FileSystemAgentFileStore("./new-memory"))

These are not drop-in replacements: the old provider uses the owner in session
state, while the new example uses session-scoped memory. Its file format and
behavior differ as described above.

Checks from python/:

• UV_NO_SYNC=1 UV_OFFLINE=1 uv run --no-sync pytest packages/core/tests/core/test_harness_memory.py packages/core/tests/core/test_harness_file_memory.py packages/core/tests/core/test_harness_agent.py::test_create_harness_agent_includes_all_default_providers packages/core/tests/core/test_root_exports.py packages/core/tests/core/test_feature_stage.py -q — passed (118 tests).
• UV_NO_SYNC=1 UV_OFFLINE=1 uv run --no-sync pytest packages/core/tests/core/test_harness_memory.py::test_memory_store_warns_when_subclassed -q — passed after the final test edit.
• UV_NO_SYNC=1 UV_OFFLINE=1 uv run --no-sync poe syntax -P core — passed.
• UV_NO_SYNC=1 UV_OFFLINE=1 uv run --no-sync poe pyright -P core — passed.
• UV_NO_SYNC=1 UV_OFFLINE=1 uv run --no-sync poe test-typing -P core — passed.

A full build and the complete unit test suite were not run.

Related Issue

None. The change follows the agreed direction to deprecate the older memory
classes; there is no linked issue.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: implementation, tests, documentation, and PR description.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.